For the crust: Preheat the oven to 400℉. Line a baking sheet with parchment paper.

On a lightly floured surface, roll the pie dough into a 13-inch circle. Transfer the crust to a 9-inch deep-dish pie plate. Tuck the edges of the crust under and crimp as desired. Thoroughly prick the bottom and sides of the crust with a fork and freeze for 30 minutes.

Line the pie crust with parchment paper or foil, then fill with pie weights or dried beans. Bake until the edges of the crust are lightly golden brown, about 20 minutes. Remove the pie weights and parchment and let the crust cool while you make the filling. Reduce the oven temperature to 375℉.

For the crumble: In a medium bowl, whisk together the flour, brown sugar, cinnamon, and salt. Using a pastry cutter or your fingers, cut the butter into the flour mixture until it forms coarse crumbs. Set aside.

For the filling: In a medium bowl, whisk together the molasses, egg, and baking soda until smooth and combined. While whisking constantly, slowly drizzle in the boiling water until fully incorporated, lightened in color, and slightly foamy, about 30 seconds to 1 minute.

Place the pie crust on the prepared baking sheet. Evenly scatter 1 cup of the crumble over the bottom of the crust, then carefully pour the filling over the top. Sprinkle the remaining crumble evenly over the filling. Bake until the filling is mostly set but still has a slight wobble in the center, 35 to 45 minutes. Let the pie cool on a rack at least 1 hour before serving. Serve with ice cream, if desired.